The OFDM technique is known to transmit digital signals at a high transmission speed (bit rate). The OFDM technique is of the multicarrier type in that the signal to be transmitted is divided into several lower speed channels, each one being transmitted on a distinct subcarrier. The subcarrier frequencies are selected such as to be mutually orthogonal to enable the separation by the receiver of the signals by which they are modulated. The OFDM symbol comprises a set of symbols modulating corresponding subcarriers and is obtained by carrying out an Inverse Discrete Fourier Transform, particularly an Inverse Fast Fourier Transform (IFFT), of a set of input symbols.
The signal resulting from applying the IFFT, after other processing (such as parallel to serial, digital to analog conversions and a low-pass filtering operation) is subjected to a radiofrequency translation in a suitable mixer and, finally, a dispatch (for example, irradiation via an antenna for wireless communication) along the transmission channel.
The main processing of the received OFDM signal is well known to those skilled in the art. Briefly, the OFDM receiver carries out the following operations: a low-frequency translation of the OFDM received, an analog to digital conversion followed by a serial to parallel conversion and a Discrete Fourier Transform (DFT) (being typically carried out by Fast Fourier Transform technique, FFT). The DFT (by transforming the signals from the time to the frequency domain) carries out the demodulation of the OFDM signal thus allowing to obtain the digital signals carrying the symbols relative to each of the subcarriers on several outputs.
The output digital signal from DFT is thus subjected to an equalization (intended to eliminate the effects of the transmission channel) and is sent to an estimator which evaluates the symbol received. The low-frequency translation carried out by the receiver provides the generation by a local oscillator of two signals being ideally of equal amplitude and in quadrature to each other, to be combined with the OFDM signal received thus producing phase I and quadrature Q components.
Practically, the local oscillator (which should operate with a frequency equal to that of the signal transmitted and hence received) generates-two signals that have not the same amplitude and are not in quadrature to each other, i.e. the local oscillator exhibits the undesired phenomenon known as the phase and gain imbalance (or amplitude). This imbalance affects the frequency translated signal and then transformed by the DFT thereby leading to an interfering term appearing between the subcarriers which, by being added to the useful signal, can hinder the evaluation of the symbol carried out by the estimator.
The article “A Novel IQ Imbalance Compensation Scheme For The Reception of OFDM Signals” di A. Schuchert, R. Hasholzner e P. Antoine—IEEE Transactions on Consumer Electronics, Vol. 47, No. 3—August 2001 (pages 313-3.18), describes a diagram of an OFDM receiver which carries out a filtration aiming at compensating the effects of the imbalance. This article provides an imbalance compensation based on the use of a FIR filter (Finite Impulsive Response) operating on the signals in the frequency domain (i.e. on those signals resulting from the DFT) according to coefficients which are adapted to the received signal variations. The initial value of these coefficients is set during a training transmission phase employing pilot subcarriers.
It has been observed that the conventional means for compensating the phase and gain imbalance in OFDM receivers, such as that referred to in the above article, do not allow a satisfactory operation. More specifically, it has been noted that the conventional techniques carrying out the compensation by using frequency domain signals do not ensure that the interference term related to the imbalance is satisfactorily eliminated.
Particularly, it is noted that these drawbacks in the art are also related to the presence of an offset between the operating frequency of the local oscillator and the frequency of the radiofrequency carrier of the received signal. It should be noted that the method of the above mentioned article by A. Schuschert et al. does not take the presence of such offset into account, therefore it reflects a non realistic situation.